1. Field of the Invention
The present invention relates to a glue applicator for applying an adhesive to the spine or adjacent areas of a book block of stacked printed sheets being conducted past the glue applicator in a transport direction, the glue applicator consisting of a tank holding liquid adhesive, into which at least one glue applicator roll dips to take up the adhesive and to transfer it to the spine of the book block, the roll being driven in the same travel direction as the book block, where the glue applicator roll works together with a doctor blade, which is installed above the surface of the glue in the tank, and where the doctor blade determines the thickness of the adhesive film to be transferred to the spine of the book block through variation of its distance from the glue applicator roll and for adjustment purposes is connected to an actuating element driven by a motorized shaft.
2. Description of the Related Art
In the production of brochures, catalogs, paperbacks, books, and book blocks by gluing, glue applicators which apply the adhesive to the spine of the book block by means of glue applicator rolls are used in most cases. For this purpose, the loose printed sheets or pages are first stacked in a stacking device. Then the spine area is processed by milling tools, or the sheets are stitched together at the spine. Finally, through the use of an adhesive, the stacked sheets are bonded to each other and to a cover or folded strip. While the spine is being processed, the glue is being applied, and the block is being assembled with the cover or folded strip, the sheets are held clamped along the sides in a transport device in such a way that that spine and the area near the spine are left free. The blocks are then conducted past stationary processing stations. The glue is applied by means of driven glue applicator rolls, the circumferential velocity of which is more-or-less the same as the transport speed of the transport device. The glue applicator rolls dip into the adhesive kept in a tank and carry the glue toward the surface to be glued. So-called doctor blades work together with the glue applicator rolls to form a transfer gap, which determines the thickness of the glue film on the glue applicator roll. If the entire length of the book block spine is not to be glued, the doctor blades are moved up and down by means of the drive devices in a direction perpendicular to the surface of the glue applicator rolls. After the glue has been applied to the spine, the adhesive film is smoothed out by an leveling device, such as a roll driven in the direction opposite that of the transport device. A glue applicator of the type indicated above is disclosed by Liebau/Heinze in Chapter 4.2.3.3.6, Industrielle Buchbinderei, 2001.
According to a first prior art, the doctor blades are opened and closed by means of radial cams, which are rotated in synchrony with the machine cycle. This solution suffers from the disadvantage that, for geometric reasons, the radial cams cannot be designed with the necessary slope, and thus the thickness of the glue film on the applicator rolls does not change quickly enough. The length of the glue film and its thickness must therefore be adjusted either by hand or by means of a complicated mechanism, which is driven by servo motors. As a result, the inertia of the system, which is already high to begin with, is increased even more, and the maximum achievable production speed is limited accordingly.
Another prior art is disclosed in EP 1 208 998 of the present applicant. The doctor blades are driven here by means of very directly connected, controllable electric motors. These drives can be easily operated and controlled by the machine control system. Direct interventions by the operator are not necessary. The disadvantage is that, to achieve the necessary resolution, the motor must travel a long distance, which results in a limitation on the maximum achievable production speed.
According to yet another prior art, it is proposed in DE 102 42 259 that the blades be driven by piezeoceramic actuators. Because of the relatively small achievable stroke of these piezeoceramic devices, a large ratio must be set between the travel of the drive and the travel of the blade, which leads to high mechanical load on the piezoelectric crystal. The maximum possible switching speed is thus severely limited by the mechanical strength of the piezoelectric crystal.
Another prior art, disclosed in DE 102 42 260, proposes the use of contraction hoses, controlled by compressed air, as actuators for the drive of the blades. It is known that, because of the compressibility of compressed air, delays occur between the switching-on of the compressed air and the reaction of the drive, the negative effects of which increase as the speed of the machine increases. Thus the maximum possible production speed is again severely limited.
The precision with which the application of the glue to the spine starts and ends depends directly on the form of the glue film present on the glue applicator rolls. Ideally, the boundaries of the glue film at the beginning and end should be surfaces which are perpendicular to the roll. This is not possible for various physical reasons. The glue film will therefore always have slopes of greater or lesser steepness at the beginning and end, the steepness of these slopes being determined by the ratio between the velocity of the blade perpendicular to the glue applicator roll and the circumferential velocity of the glue applicator roll. Because the circumferential velocity of the glue applicator rolls is determined by the spacing between books and the production speed, the necessary steepness and accuracy of the slopes can be achieved only by blades which are able to move at appropriately high speeds.